Note 107 — Chapter VI · OF THE PARTICIPLE.
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This verb is now used as significant of present duty. It was originally the preterite, and the perfect participle of the verb to owe; and is corruptedly used in Scotland still to express a past debt. “Apprehending the occasion, I will add a continuance to that happy motion, and besides give you some tribute of the love and duty I long have ought you.”—Spelman.
“This blood, which men by treason sought, That followed, sir, which to myself I ought.”—Dryden.
It is now used in the present tense only; and, when past duty or obligation is to be signified, we note, as I formerly mentioned, the past time by the preterite sense of the subsequent verb; thus, “I ought to read,” “I ought to have read.” The classical scholar knows that the reverse takes place in Latin. Debeo legere, debui legere. Cicero, however, though very rarely indeed, uses the preterite of the infinitive after the preterite tense of this verb.
Murray has told us, that must and ought have both a present and past signification, and, in proof of this, he adduces the following examples:—“I must own, that I am to blame.” “He must have been mistaken.” “Speaking things which they ought not.” “These ought ye to have done.” This is truly a strange, and, I verily believe, a singular opinion. Its inaccuracy is so manifest, that every reader of discernment must intuitively perceive it. The opinion itself, indeed, is not more surprising, than the ground on which it is maintained by the author. It surely requires but a moderate portion of sagacity to perceive, that the past time, in the second and fourth examples, is not denoted by must and ought, but by the expressions “have been” and “have done.” In Latin, as I have just observed, necessity and duty are expressed as either present, past, or future, the verbs denoting these having the three correspondent tenses; and the object of the necessity or duty is expressed as contemporary, or relatively present. In English, on the contrary, the two verbs must and ought having only the present tense, we are obliged to note the past time by employing the preterite tense of the subsequent verb. Thus, Me ire oportet, “I ought to go,” “I must go.” Me ire oportuit, “I ought to have gone,” “I must have gone.” As well may it be affirmed, that the past time is denoted by ire and not oportuit, as that it is signified by must and not by “have gone.”
In the time of Wallis, the term must, as a preterite tense, was almost obsolete. “Aliquando,” he remarks, “sed rarius in præterito dicitur.” And when it was employed as a preterite, it was followed by the present tense. This verb in German has, I understand, a preterite tense.